The present invention relates to a speed measuring apparatus capable of detecting the speed of a vehicle powered by an internal combustion engine with high accuracy.
For detecting the speed of a vehicle with an internal combustion engine, there has been widely used a speed detecting apparatus employing a multi-poled magnet operatively coupled through a speedometer capable with the output shaft of a rotational power transmitting device and a reed switch which is opened/closed in accordance with the rotation of the magnet. However, in the conventional speed detecting apparatus, the distances between the magnetic poles of the magnet are not uniform. Consequently, when the engine or other device associated with the vehicle is controlled in a digital manner on the basis of the pulse signal produced by the opening/closing operation of the reed switch, because of the non-uniformity of the intervals between the magnetic poles, hunting is apt to occur in the vehicle speed control operation and deviation from a desired target value is also apt to arise. As a result, the vehicle speed control accuracy is lowered.
To overcome these disadvantages, it is necessary to use a magnet in which the magnetic poles are uniformly arranged. However, magnets with equidistant poles are expensive so that their use will increase the cost of the speed detecting device. In this connection, if the vehicle speed is computed on the basis of the period for one rotation of the magnet, the computed vehicle speed will be unaffected by the non-uniform spacing of the magnetic poles. However, in this case since the period over which the vehicle speed is detected becomes long, it is impossible to quickly carry out the desired control operation in a digital manner and, as a result, the control accuracy is lowered.
Accordingly to one known approach, there is proposed a speed detecting device for eliminating these drawbacks. In the proposed device the gear ratio set in the transmission is mechanically detected and the vehicle speed is calculated on the basis of the detected gear ratio and the engine speed at each instant. However, since the gear ratio selected in the transmission is mechanically detected by the use of a mechanical device in the proposed speed detecting device, mechanical switches for sensing the position of the shift lever must be provided in the transmission. Consequently, the proposed device is disadvantageous because of its mechanical complexity and high cost.